The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without payment of any royalties thereon or therefor.
1. Field of the Invention
The present invention relates to analog-to-digital converters (ADC) and, more particularly, to an architecture for faster optical analog-to-digital conversion with larger bandwidth and a technique for incorporating the architecture in electronic devices.
2. Description of the Background
Electronic conversion of analog signals to digital format is a well-established technology, but the constant, strong consumer demand for faster speeds has created a demand for faster optical analog to digital (A/D) converters to provide higher sampling rates and allow conversion of ever-higher frequency analog signals.
A variety of optical (or xe2x80x9cphotonicxe2x80x9d) A/D converters have evolved more recently to satisfy the need, and existing photonic A/D converters have taken a variety of forms.
One known approach to high-speed analog-to-digital conversion uses an electrooptical modulator for optical sampling. See H. F. Taylor, xe2x80x9cAn Electrooptic Analog-to-Digital Converter, Proceedings of the IEEE, Volume 63, Pages 1524-1525, 1975. Taylor""s electrooptic A/D converter uses an electrooptic modulator with an output intensity that has a periodic dependence on the voltage applied to its modulation input. The output of the modulator is captured by a photodetector and compared to a threshold signal. The result of the comparison is a digital output that represents an analog-to-digital conversion of the analog voltage applied to the modulation input. Taylor also discloses a multibit A/D converter that uses multiple modulators where the ratio of the output intensity to the input voltage of each modulator is progressively scaled by a factor of two. However, the speed of Taylor""s electrooptic A/D converter is limited by the speed of the electronic amplifiers and comparators that compare the modulator outputs to the threshold signal.
Subsequent efforts have attempted to increase the speed by replacing the electronic circuitry with optical circuitry. For example, U.S. Pat. No. 6,420,985 to Toughlian et al. issued Jul. 16, 2002 shows a photonic wide-band analog to digital converter in which an analog electrical signal is first converted to an optical signal having a wavelength that is a function of the amplitude of the analog electrical signal. The optical signal is then filtered in a plurality of optical filter channels to create N optical bit signals forming an N bit binary word indicating the wavelength of said optical signal. These optical bit signals are then each converted to an electrical bit signal to form an electrical binary word. Toughlian et al. ""985 eliminates all electronics by using a tunable laser to convert the electrical signal to an optical signal having a wavelength that is a function of the amplitude of the analog electrical signal. As with Toughlian ""985, other known devices tend to split or divide the stream of pulses into a number of streams proportional to the resolution sought, and then they attenuate the paths by a different amount. See, for example, U.S. Pat. No. 6,326,910 issued to Hayduk et al. on Dec. 4, 2001.
U.S. Pat. No. 6,404,366 to Clark et al. issued Jun. 11, 2002, shows a photonic analog-to-digital converter that uses wavelength division multiplexing and distributed optical phase modulation. The analog-to-digital conversion is performed within the optical system of the ADC and thus facilitates ADC conversion at much higher speeds than available with conventional electronic ADCs. First, a plurality of optical signals of differing wavelengths is produced using a multiwave length optical source. These multiwave length signals are passed through a polarizer in order to set an initial polarization state in the ADC system. The polarized optical signals are passed through an electrooptic modulator that modifies the polarization states of the optical signals. This way, the change in polarization is converted to a change in optical intensity, which can be used to produce an individual binary optical output representing a most significant bit in the digitized representation of the analog signal. The modified optical signal is then passed through a wavelength filter, disposed in between a plurality of electro-optic modulators, in order to extract an optical signal of a specified wavelength. The unextracted optical signals are passed through a second electrooptic modulator to produce a modified optical signal, and the modified optical signal is passed through a second wavelength filter to extract an optical signal of another specified wavelength. The extracted signal is likewise processed to produce an individual binary optical output. The above method continues until signals of all the wavelengths are extracted and processed resulting in binary optical outputs. The combination of all the binary optical outputs produces a digital equivalent value of the analog signal.
U.S. Pat. No. 6,188,342 issued to Gallo on Feb. 13, 2001 shows a photonic A/D converter using parallel synchronous quantization of optical signals. The converter divides the input optical signal into N channels such that each channel is provided progressively more optical power from most to least significant bit on a predetermined scale. This is accomplished by giving each channel a different number of photodetectors, such that the most significant channels have the least number of photodetectors. The channel with the least sensitivity (that is, the fewest number of photodetectors) determines the most significant bit in the quantization of the lightwave signal. This allows a division of the input analog optical signal whereby a small fraction of the signal is used to determine the most significant bit and greater portions of the input signal are directed to each lesser significant bit.
U.S. Pat. No. 6,404,365 issued to Heflinger on Jun. 11, 2002 shows a fully optical analog to digital converter with complementary outputs. The Heflinger ""365 patent uses a Mach-Zehnder interferometer, which splits the light from each path into two split out interferometer paths and then combines the two paths to generate two complimentary output signals. When the two paths are combined, optical interference directs the light proportionally into the two complimentary output signals. In operation, the output of each interferometer is a complementary sinusoidal variation in the intensity partitioned between the two output signals. Thus, the interferometer for the most significant bit delivers just one cycle of variation in intensity. The interferometer for the next most significant bit experiences two complete cycles, etc.
Although Heflinger ""365 generally suggests splitting and recombining the paths to vary the bit significance, the interferometer approach is relatively expensive and space consuming. There remains a need for a high resolution, low power approach that is capable of implementation in a small package, e.g., that can fit on an integrated circuit.
It is, therefore, a desired feature of an A/D converter architecture of the present invention that each path be attenuated by a different amount (attenuates the different paths along a gradient), then splits each path again and recombines each one with an adjacent path in such a way that only one path has significant energy.
According to the present invention, the above described and other features are accomplished by a method of photonic analog-to-digital conversion including the steps of using an analog signal to modulate a laser, splitting the modulated optical output into N paths, attenuating the different paths along a gradient, then splitting each path again and recombining with an adjacent path in such a way that only one path has significant energy. Implementation architecture is also provided and this is comprised of a laser source that is modulated into 2N paths (where N is the number of bits of resolution). Each of the paths is attenuated by a different amount, and then each path is split again and recombined with an adjacent path. The lines are recombined in such a way that only one path has significant energy. This signal is fed to a decoder that directs the light to the proper photodiodes, where it is converted to a binary word.
The present invention""s method and implementation architecture is simple and, straightforward, and can be economically manufactured using standard integrated circuit fabrication technology.